Hard metal (tungsten carbide), wafer-like inserts of the indexable, replaceable (throwaway) type have been used in milling cutters for a number of years, as well as in single point tools. Boring bars have also used these inserts carried in removable cartridges as exemplified in U.S. Pat. No. 3,152,493, to Sullivan, dated Oct. 13, 1964. A milling cutter with replaceable inserts is shown in a U.S. Pat. No. 3,757,397, to Lindsey, dated Sept. 11, 1973. Another example of a boring bar is shown in a U.S. Pat. No. 4,030,843, to Harp, dated June 21, 1977, where the inserts are clamped in axial side slots. A drill using throwaway inserts is shown in a U.S. Pat. No. 3,540,323, to Rishel, dated Nov. 17, 1970. This patent shows the use of a central retainer pin which pushes the insert into contact with a retaining pocket.
The above patent to Rishel illustrates what is believed to be a typical approach to a replaceable insert drill where the inserts are spaced radially to cut an equal amount diametrically of the hole.
The concept of the present invention for a replaceable insert drill lies in the selection, positioning and shaping of the inserts on the drill body so that the cutting action of each tooth is equalized. The amount of work done by each tooth is by design and location such that the volume of chip removal for each tooth per revolution is approximately equal. For example, as a particular hole gets larger in diameter, the circumference increases, causing teeth at the larger diameters to remove more material than those at the smaller diameter for each revolution of the drill. With the present invention, the volume of material to be removed by each tooth is equalized. The torque in the drill body is more evenly divided with this arrangement, thus removing one of the causes of chatter. Also, the volume of chips from each blade is equal, thus making chip removal easier.
The present invention also contemplates a tool which can be used as a boring bar finishing tool by simply shifting it radially a small amount. In an automatic tool set up, this has many advantages relative to time by reducing the required number of tool stations and the like.
Other features of the invention include the fastening means for the inserts and other details will be found in the following description and claims in which the principles of the invention are further set forth together with details to enable a person skilled in the art to practice the invention, all in connection with the best mode presently contemplated for the invention.